DESCRIPTION (Applicant's Abstract): The purpose of this study is to determine whether gamma knife radiosurgery is effective in reducing or eliminating seizures in patients with temporal lobe epilepsy. The primary hypothesis is that radiosurgical treatment of mesial temporal lobe structures, including the amygdala and hippocampus, will result in a significant reduction in seizures in patients with medically refractory medial temporal lobe epilepsy with minimal morbidity to patients. Patients who are refractory to medical management (20% of epilepsy patients) may be referred for surgical management. Approximately half of them are found to be candidates for surgical resection of their seizure focus, most commonly by temporal lobectomy. Open surgical procedures have inherent risks including damage to the brain, bleeding, and infection. Radiosurgery is a technique whereby gamma radiation is focused (201 sources) using stereotaxic guidance into precisely controlled regions of the brain. The main risks of this procedure are radiation injury to important nearby structures, and radiation necrosis. There is preliminary evidence that high-dose radiation reduces seizures, but long-term outcome is available in very few cases. The optimal radiation dose, the minimum effective treatment volume, and the true morbidity of this approach have not been established. This protocol defines a homogeneous group of patients with mesial temporal lobe epilepsy in whom the outcome from open surgery is well established. It will compare two radiosurgical doses directed at the medial temporal lobe to standard medical therapy. Radiosurgical treatment will be standardized over all Treatment Centers using inter-center data transfer unique to the Gamma Knife instrument. Follow-up data on seizure frequency (including seizure-free outcome), neuropsychological testing, MRI (including Perfusion and Diffusion weighted studies), MRS findings and EEG changes will be collected for three years following radiosurgical treatment. The pilot data obtained from this protocol will result in a better definition of the safe and effective radiation dose for use in future protocols including a comparison of radiosurgery to temporal lobectomy. If radiosurgery can be shown to reduce or eliminate seizures in patients with medically refractory temporal lobe epilepsy, patients could be offered this minimally invasive technique, avoiding the morbidity of open cranial surgery.